The reason the timing can effect dieseling is because if out of spec too far it creates excessive heat . Anything that causes excessive heat or is glowing when the engine is shut down will ignite the fuel ( plugs,carbon,casting,valves,etc.) and result in dieseling, even without spark. I am not familar with an '86 454 in particular but some engines have an anti-dieseling device on the carburator to shut down the butterfly to prevent this . If so equiped , perhaps this device is not functioning properly.

Update: I went to the gas station and put the highest octane gas in Ol' Nancy and have been shutting her down with it in drive or park and the "dieseling" has subsided for the moment. thanks for all the helpful comments. You guys are the best.

Update: I went to the gas station and put the highest octane gas in Ol' Nancy and have been shutting her down with it in drive or park and the "dieseling" has subsided for the moment. thanks for all the helpful comments. You guys are the best.

I found that heater on the electric choke was bad on Nancy causing the idle to stay "high" and not drop down to "curb Idle speed". Replaced choke and that cured the dieseling.